Common Stock and Stockholders' Equity - Capitalization (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 04, 2017	Jan. 24, 2017	Mar. 31, 2018	Dec. 31, 2017	May 25, 2017	May 24, 2017
Capitalization
Number of common stock shares authorized			100,000,000	100,000,000	100,000,000	75,000,000
Follow-On Offering
Capitalization
Issuance of stock (in shares)	4,096,600	7,887,500
Price of stock issued (in dollars per share)	$ 35.00	$ 16.75
Proceeds received, net of issuance costs	$ 134.7	$ 123.9
Underwriting discounts and commissions	8.2	7.6
Other offering expenses	$ 0.5	$ 0.6